A Reassessment of the Taxonomic Position of Mesosaurs Based on Two Data Matrices

1The Early Permian mesosaurs are the oldest known primarily aquatic amniotes. Despite the interest that they have generated over time, their affinities remain controversial. Recently, two hypotheses have been supported, in which mesosaurs are either the sister-group of all other sauropsids, or the sister-group of other parareptiles. We recently upheld the former hypothesis, but in the latest study on mesosaur affinities, MacDougall et al. published a study highly critical of our work, while upholding the hypothesis that mesosaurs are basal parareptiles. We expect that the debate about mesosaur affinities will continue in the foreseeable future, but we wish to respond to the two central comments published by MacDougall et al. in 2018, who argue that variability in the temporal fenestration of early sauropsids, combined with the omission of several recently-described parareptile taxa, explain the differences in topologies between their study and ours. Reanalyzing our data matrix and theirs without characters linked with temporal fenestration, and removing from their matrix the parareptile taxa that they added (and that we omitted) does not alter the resulting topologies. Thus, their main conclusions are false; the differences in taxonomic position of mesosaurs must result from character choice and scoring differences.